In conventional heat exchangers, in order to improve drain performance of condensed water and improve fin thermal conductivity, it has been proposed to “form a drain groove (10) for guiding condensed water downward at a middle portion in an air flow direction (A) on a tube (2) having a flat cross sectional shape and extending in a vertical direction and a gap portion (53) at a position which faces the drain groove (10) on a corrugated fin (5) which is joined to the outer wall of the tube (2) and folded in a meandering shape so that the corrugated fin (5) is divided by the gap portion (53) into an upstream-side first fin (51) and a downstream-side second fin (52)” (for example, see Patent Literature 1).